We report the occurrence of cytopenia and hypothyroxinemia attributed to decreased levels of thyroxine-binding globulin in patients receiving nicotinic acid. We describe two patients in whom hypothyroxinemia developed while they were taking nicotinic acid; these patients also had decreased levels of thyroxine-binding globulin. Results of all thyroid function tests returned to normal when use of the nicotinic acid was discontinued. In one patient, leukopenia and thrombocytopenia developed during nicotinic acid therapy alone. These conditions were reversed after the drug regimen was discontinued. In another patient, leukopenia and thrombocytopenia developed during combination nicotinic acid and lovastatin therapy. When administration of both drugs was discontinued, the hematologic abnormalities, which could have been due to either nicotinic acid or lovastatin, diminished. We suggest that cytopenia may develop in patients receiving nicotinic acid; thus, thyroid function tests should be interpreted in light of a possible decreased level of thyroxine-binding globulin.